In general, the field of the present invention is blastocysts. Specifically, the field of the present invention is composite blastocysts formed following dissociation, selection, and aggregation of cells derived from discarded pre-embryos.
During therapeutic in vitro fertilization, several eggs (15 on average) are collected from the female patient""s ovaries following stimulation by exogenous gonadotrophins. Following retrieval, the eggs are incubated for 4-6 hours then inseminated with sperm obtained from the male partner. Approximately 16 hours following the insemination procedure, eggs are checked for evidence of fertilization; fertilization is confirmed when two pronuclei, one originating from each gamete donor, are visualized in the egg. At this stage, the entity is a single cell called a zygote. The zygote is then cultured for an additional two days during which it typically undergoes three cleavage divisions, yielding an eight-cell pre-embryo. It is typically at this stage of development that pre-embryos are replaced into the uterine cavity in hopes of establishing a pregnancy. Approximately 25% of the replaced pre-embryos develop into a fetus. Pre-embryos that are not replaced are frozen, for later therapeutic use, if they are normal in morphology or (in case of preimplantation genetic diagnosis) cytogenetically. If they are morphologically or cytogenetically abnormal, they are considered non-viable and are therefore discarded.
A multitude of studies over the past two decades have identified cleavage rate, cell number, cleavage symmetry, cytoplasmic fragmentation, blastomere nucleation, zona pellucida structure, degree of cell-cell contact, and cytoplasmic organization as key markers of viability in fresh human pre-embryos. The viability of frozen-thawed cleavage stage pre-embryos is judged, in addition, by the proportion of cells that remain intact after thawing.
Development is severely compromised in fresh or frozen/thawed pre-embryos with the following characteristics, singly or in combination: 1) Fewer than 2 cells on day 2 of development; 2) Fewer than 5 cells on day 3 of development; 3) No division in 24 hours of culture; 4) One or more highly uneven cleavage divisions; 5) Loss of 35% or more of total cytoplasmic volume to fragmentation or degeneration; 6) Large fragments distributed throughout the pre-embryo, 7) One or more multinucleated blastomeres appearing either on day 2 or day 3 of development; 8) Less than 50% of the cells intact after thawing of cryopreserved pre-embryos (Fresh pre-embryo morphology is reviewed by Alikani et al., Human Embryo Morphology and Developmental Capacity, In: Mammalian Embryo Quality Evaluation, Invasive and Non-invasive Techniques, Ann Van Soom and Marlene Bjorn, eds., Kluwer""s Academic Publishers, 2002, in press; Frozen-thawed pre-embryo development capacity is analyzed by Edgar D H et al., A quantitative analysis of the impact of cryopreservation on the implantation potential of human early cleavage stage embryos. Human Reproduction 15: 175-179, 2000).
Blastocysts typically develop on the fifth or sixth day of pre-embryo culture in vitro. At this time, they typically contain from 50 to over 100 cells. A blastocyst is composed of two tissue types 1) a distinct inner cell mass (ICM) placed eccentrically in the blastocoelic cavity, and a single-cell layer of enclosing trophectoderm (TE). Cells of the ICM and TE can be differentially stained using bisbenzimide and propidium iodide, respectively (Thouas et al., Simplified technique for differential staining of inner cell mass and trophectoderm cells of mouse and bovine blastocysts, Reproductive Bio Medicine On Line; web paper, Vol. 3, No. 1, 25-29, 2001). While 70% of eggs fertilized in-vitro typically undergo the first three cleavage divisions during three days in culture, less than 50% advance to cavitation after five days (Gardner et al., A prospective randomized trial of blastocyst culture and transfer in in-vitro fertilization. Human Reproduction, 13:3434-3440, 1998), and about one third of those blastulating form morphologically normal blastocysts that expand to fill the zona pellucida completely and have a well defined ICM, and a cohesive TE. Pre-embryos with the abnormalities mentioned above hardly ever form blastcysts even under optimal culture conditions (Alikani et al., Cleavage anomalies in early human embryos and survival after prolonged culture in vitro, Human Reproduction, 15:2634-2643, 2000). These abnormal pre-embryos are therefore discarded.
One patent referring to blastocysts, U.S. Pat. No. 5,541,081, discusses the use of biochemical markers. The use of blastocysts for deriving embryonic stem (ES) cells is discussed in U.S. Pat. Nos. 5,843,780, 6,200,806, 628,971 which are incorporated by reference.
In some mammals, composite blastocysts (CBs) have been produced experimentally by combining two or more whole embryos or cells derived from two or more embryos (McLaren A and Bowman P. Mouse chimaeras derived from fusion of embryos differing by nine genetic factors. Nature 224:238-240; 1969)
The production and use of CBs from cells derived from normal embryos are discussed by Fehilly CB et al. (Interspecific chimaerism between sheep and goat, Nature 307: 634-636, 1984) and, Experimental chimaerism in sheep, J Reproduction and Fertility 70:347, 1984). No reference exists in the literature to either the production or the potential uses of CBs from non-viable, zygote-derived pre-embryos within a species.
CBs could be used to produce TE- and/or ICM-derived ES cells and cell lines. The latter have a vast number of clinical and scientific uses (NIH website on stem cells).
Present day embryo-based therapeutics and the state of the art clinical embryology pre-suppose the use of viable, potentially implantable pre-embryos. Pre-embryos with abnormal morphology are assumed by investigators of ordinary, as well as expert, skill in the art to have aberrant component cells (Jurisicova et al., Programmed cell death and human embryo fragmentation. Molecular Human Reproduction, 2: 93-98, 1996; Antczak and Van Blerkom, Temporal and spatial aspects of fragmentation in early human embryos: possible effects on developmental competence and association with the differential elimination of regulatory proteins from polarized domains. Human Reproduction, 14: 429-447, 1999).
Therefore the use of components from discarded abnormal pre-embryos is not obvious to those of ordinary, as well as expert, skill in the art. Nor has this approach been discussed at embryology conferences or published or patented. This invention, therefore, is novel and in addition represents a paradigm shift in the area of embryology.
This invention is a product and a process. The product is a composite blastocyst (CB). The process is an aggregation process (AP). The material for both the process and the product is non-viable discarded pre-embryos. The CB is characterized by a cellular morphology having two distinct tissue types, the inner cell mass (ICM) stainable with bisbenzimide, and the trophectoderm (TE) stainable with propidium iodide, and a blastocoelic cavity (BC). The ICM contains embryonic stem (ES) cells, which are pluripotent. The primate TE is characterized by the production of chorionic gonadotrophin. The method of making CBs is an AP comprising inter alia the following steps: 1) dissociation of non-viable discarded pre-embryos; 2) selection of single-nucleated cells from several dissociated discarded pre-embryos; 3) microsurgical placement of multiple cells within a host zona pellucida or artificial aggregation in a non-zona vessel; 5) primary culture of the aggregates for multiplication and differentiation of cells.
The CB can be used to make
I) ES cell lines. True ES cell lines are characterized as follows:
i) they are capable of indefinite proliferation in vitro in an undifferentiated state;
ii) they are capable of differentiation into derivatives of all three embryonic germ layers (endoderm, mesoderm, and ectoderm) even after prolonged culture;
iii) they maintain a normal karyotype throughout prolonged culture. The true primate ES cell lines are therefore pluripotent.
II) TE derived stem (TES) cells.
Historically, the production of ES and TES cells has required the use of viable pre-embryos and their maintenance in culture until the development of a blastocyst containing an ICM and a TE. The source of viable human pre-embryos has been either unwanted frozen embryos donated to research by couples who have already undergone an IVF treatment cycle (Thomson et al. Embryonic stem cell lines derived from human blastocysts, Science 282:1145-1147,1998) or embryos created specifically for the purpose of stem cell production using gamete donors (Lanzendorf et al., Use of human gametes obtained from anonymous donors for the production of human embryonic stem cell lines. Fertility and Sterility 76: 132-137, 2001). Neither of these options is regarded acceptable by the federal government of the United States since they involve the destruction of viable pre-embryos; human embryonic stem cell research is therefore not federally funded unless the cell lines were made prior to Aug. 9, 2001(Fletcher L. US Stem cell policy comes under fire. Nature Biotechnology, 19: 893-894, 2001).
The present invention provides an alternative to the use of viable pre-embryos for these purposes, that is, the use of non-viable pre-embryos.
Three lines of evidence have led us to this invention:
1) each component cell within a normal mammalian embryo was found to maintain its totipotency at least through the first three cleavage divisions (Tarkowski A K and Wroblewska J. Development of blastomeres of mouse eggs isolated at the 4- and 8-cell stage, J Embryology and Experimental Morphology, 18(1):155-80,1967; Willadsen S M and Polge C, Attempts to produce monozygotic quadruplets in cattle by blastomere separation. Veterinary Record 108:211-3, 1981; Willadsen S M, The development capacity of blastomeres from 4- and 8-cell sheep embryos, J Embryology and Experimental Morphology 65:165-172, 1981; Willadsen S M, The viability of early cleavage stages containing half the normal number of blastomeres in the sheep. J Reproduction and Fertility, 59: 357-62, 1980)
2) chimeric embryos produced from normal cells isolated from viable embryos were capable of full development (Fehilly C B et al., Experimental chimaerism in sheep, J Reproduction and Fertility 70:347, 1984)
3) while non-viable discarded pre-embryos either arrest in extended culture or undergo abnormal development (Alikani et al., Cleavage anomalies in early human embryos and survival after prolonged culture in vitro, Human Reproduction, 15:2634-2643, 2000), a large proportion of component cells from such non-viable pre-embryos undergo one or two divisions and proceed to cavitation when cultured in isolation (Alikani, unpublished observations).
The present invention is also summarized in that CBs are morphologically similar to ordinary blastocysts. The CBs have a morphologically identifiable inner cell mass (ICM) and trophectoderm (TE). It is an advantageous feature of the present invention that the CB mimics the ordinary blastocyst in terms of its complex morphological development. Therefore, the present invention inter alia allows cells from discarded pre-embryos to serve as a source of human embryonic and trophectodermic stem cells without the need to create new pre-embryos or destroy any existing viable pre-embryos.
It is an object of the invention to provide the material for producing primate ES cell lines without creation of new embryos or destruction of existing viable and/or implantable embryos.
It is an object of the invention to provide a primate CB that can be the material for creating ES cells without creation of new embryos or destruction of viable and/or implantable embryos.
It is another object of the present invention to provide the material for producing primate embryonic stem cell lines wherein the cells differentiate into cells derived from mesoderm, endoderm, and ectoderm germ layers without creation of new embryos or destruction of viable embryos.
Other objects, features, and advantages of the present invention will become obvious after study of the specification, drawings, and claims.